Contact for a semiconductor light emitting device

ABSTRACT

Embodiments of the invention include a semiconductor structure comprising a III-nitride light emitting layer disposed between an n-type region and a p-type region. A contact disposed on the p-type region includes a transparent conductive material in direct contact with the p-type region, a reflective metal layer, and a transparent insulating material disposed between the transparent conductive layer and the reflective metal layer. In a plurality of openings in the transparent insulating material, the transparent conductive material is in direct contact with the reflective metal layer.

FIELD OF INVENTION

The present invention relates to a reflective contact for a III-nitride light emitting device.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, composite, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions. III-nitride devices are often formed as inverted or flip chip devices, where both the n- and p-contacts formed on the same side of the semiconductor structure, and light is extracted from the side of the semiconductor structure opposite the contacts.

U.S. Pat. No. 6,514,782 describes III-nitride flip chip LEDs. “Because of the high resistivity of p-type III-nitride layers, LED designs employ metallization along the p-type layers to provide p-side current spreading . . . For an inverted design, using highly reflective electrode metallizations is critical to improve the extraction efficiency . . . The p electrode is the dominant factor for light extraction because it extends almost completely across the active area to provide uniform current injection into the p-n junction.

“The combination of low optical absorption and low contact resistivity in a manufacturable process are difficult to achieve for III-nitride devices. For example, Ag makes a good p-type Ohmic contact and is very reflective, but suffers from poor adhesion to III-nitride layers and from susceptibility to electro-migration in humid environments which can lead to catastrophic device failure. Al is reasonably reflective but does not make good Ohmic contact to p-type III-nitride materials, while other elemental metals are fairly absorbing (>25% absorption per pass in the visible wavelength regime). A possible solution is to use a multi-layer contact which includes a very thin semi-transparent Ohmic contact in conjunction with a thick reflective layer which acts as a current spreading layer. An optional barrier layer is included between the Ohmic layer and the reflective layer. One example of a p-type multi-layer contact is Au/NiO_(x)/Al. Typical thicknesses for this metallization scheme are 30/100/1500 Å. Similarly, a suitable n-type GaN multi-layer contact is Ti/Al with typical thicknesses of 30/1500 Å. Since the p-electrode reflectivity is a dominant factor in extraction efficiency, it must not be compromised in designing for manufacturability.”

SUMMARY

It is an object of the present invention to include in a reflective contact a thin, transparent current spreading layer and a transparent insulating material. In some embodiments, reflectivity of the contact may be improved over a device with a reflective metal contact.

Embodiments of the invention include a semiconductor structure comprising a III-nitride light emitting layer disposed between an n-type region and a p-type region. A contact disposed on the p-type region includes a transparent conductive material in direct contact with the p-type region, a reflective metal layer, and a transparent insulating material disposed between the transparent conductive layer and the reflective metal layer. In a plurality of openings in the transparent insulating material, the transparent conductive material is in direct contact with the reflective metal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a contact including a conductive layer, a transparent insulating or low-loss layer, and a reflective layer, formed on a III-nitride semiconductor structure.

FIG. 2 is a plan view of a portion of a contact including a low-loss layer with openings.

FIG. 3 is a cross sectional view of an LED bonded to a mount.

FIG. 4 illustrates a contact with openings in the transparent insulating material that extend into the transparent conductive material, and have angled sidewalls.

DETAILED DESCRIPTION

The performance of an LED may be improved by reducing optical loss associated with the p-contact, without increasing the forward voltage V_(f) required to forward bias the LED. A contact including a dielectric layer, which reflects by total internal reflection, may be more reflective than a contact where the sole reflective material is a metal reflector, such as the contacts described above in U.S. Pat. No. 6,514,782.

A conductive dielectric layer such as indium tin oxide (ITO) may be disposed between the p-type material and the silver p-contact. In such a bilayer, the ITO need not contribute to current spreading and the thickness may be optimized for highest reflectance; for example, the ITO may be 200 nm thick. However, ITO has an index of refraction higher than optimum for optical reflectivity and may absorb a significant amount of light at thicknesses required for high conductivity.

Alternatively, a nonconductive dielectric such as SiO2 may be disposed between the p-type material and the silver p-contact. Openings must be formed in the nonconductive dielectric to electrically connect the silver to the p-type material. The openings must be spaced close enough together to prevent current crowding in the poorly conductive p-type material. For example, the openings may be sub-micron size, which may require expensive and difficult techniques such as holographic or e-beam exposure of resist, or the use of a nano-imprinting tool. In addition, etching openings in the dielectric may damage the exposed p-type material, which may reduce the efficiency of an Ohmic contact formed on the damaged material.

In embodiments of the invention, the p-contact of a III-nitride LED includes three layers: a thin conductive layer in direct contact with the p-type semiconductor, a low-optical-loss dielectric layer disposed over the thin conductive layer with openings to facilitate electrical contact, and a reflective metal layer over the transparent dielectric layer.

FIG. 1 illustrates a portion of a III-nitride device according to embodiments of the invention. In FIG. 1, a semiconductor structure including an n-type region, a light emitting or active region, and a p-type region is grown over a growth substrate (not shown in FIG. 1), which may be any suitable growth substrate and which is typically sapphire or SiC. An n-type region 22 is grown first over the substrate. N-type region 22 may include multiple layers of different compositions and dopant concentration including, for example, preparation layers such as buffer layers or nucleation layers, which may be n-type or not intentionally doped, release layers designed to facilitate later release of the growth substrate or thinning of the semiconductor structure after substrate removal, and n- or even p-type device layers designed for particular optical or electrical properties desirable for the light emitting region to efficiently emit light.

A light emitting or active region 24 is grown over n-type region 22. Examples of suitable light emitting regions include a single thick or thin light emitting layer, or a multiple quantum well light emitting region including multiple thin or thick quantum well light emitting layers separated by barrier layers. For example, a multiple quantum well light emitting region may include multiple light emitting layers, each with a thickness of 25 Å or less, separated by barriers, each with a thickness of 100 Å or less. In some embodiments, the thickness of each of the light emitting layers in the device is thicker than 50 Å.

A p-type region 26 is grown over light emitting region 24. Like the n-type region, the p-type region may include multiple layers of different composition, thickness, and dopant concentration, including layers that are not intentionally doped, or n-type layers.

A thin conductive layer 28 is formed over p-type region 26. Thin conductive layer 28 may be, for example, silver, aluminum, or a conductive dielectric such as ITO, nickel oxide, ZnO, or any other suitable semitransparent conductive material. A silver conductive layer 28 may be, for example, between 0.5 and 2 nm thick in some embodiments, between 2 and 8 nm thick in some embodiments, and 10 nm thick in some embodiments. A conductive layer 28 that is a transparent conductive oxide may be thicker. For example, the resistivity of an ITO conductive layer 28 may be 100 times greater than silver, requiring an ITO conductive layer 28 that is 100 times thicker than a silver conductive layer 28. To spread current several microns may require, for example, an ITO conductive layer 28 that is 200 nm thick. The material and thickness of conductive layer 28 may be selected such that current spreads in conductive layer 28 for 10 microns, for example.

In some embodiments, thin conductive layer 28 is formed as a group of small regions, rather than as a single, uninterrupted, continuous layer. In some embodiments, a thin layer of silver is evaporated on the surface of the p-type region 26, then annealed. During the anneal, the silver tends to agglomerate from a continuous, planar layer into a network of thicker, discrete regions. For example, ten Angstroms of silver may be evaporated on to the p-type region 26. After annealing, the silver regions may be, for example, about 200 Angstroms long and about 200 Angstroms thick. The silver regions may be, for example, up to 1 micron apart in some embodiments and up to 500 nm apart in some embodiments, such that less than 10% of the surface of the p-type region 26 is covered with silver in some embodiments. In some embodiments, a planar thin conductive layer 28 may be formed, then etched to form a group of small regions. In some embodiments, during silver deposition the structure is heated to encourage migration and agglomeration of silver into a network of thicker, discrete regions.

A low optical loss material 30 is formed over conductive layer 28. Low-loss material 30 may be, for example, SiO_(x), SiN_(x), MgF₂, Al₂O₃, or any other suitable highly transparent dielectric that is reflective, manufacturable, and readily adheres to the conductive layer 28. In some embodiments, low-loss material 30 has a low index of refraction, so the change in index of refraction between the low-loss material and the conductive layer 28 and p-type region 26 is as large as possible. Low-loss material 30 may be, for example, between 200 and 500 nm thick in some embodiments, between 250 and 350 nm thick in some embodiments, and 250 nm thick in some embodiments.

Openings 32 are then formed in the low-loss material 30, for example by conventional masking and etching steps. In some embodiments, endpoint detection is used to avoid etching the underlying conductive layer 28. In some embodiments, the low-loss material 30 is etched to near the endpoint with a dry etch, then a wet etch is used to etch the remaining thickness. FIG. 2 is a plan view of the structure of FIG. 1 after forming openings 32 in low-loss material 30. Though round openings formed in a triangular lattice are illustrated, any suitably-shaped openings in any suitable lattice may be used. Openings 32 may be, for example, less than 100 microns in diameter in some embodiments, between 1 and 5 microns in diameter in some embodiments, between 2 and 15 microns in diameter in some embodiments, between 2 and 4 microns in diameter in some embodiments, and 3 microns in diameter in some embodiments. The openings may be spaced, for example, between 20 and 200 microns apart, on centers between 5 and 20 microns apart in some embodiments, between 10 and 15 microns apart in some embodiments, 6 microns apart in some embodiments, and 12 microns apart in some embodiments.

The size and spacing of the openings may be related to the resistivity and thickness of conductive layer 28. For example, a 150 nm thick layer of ITO has about the same sheet resistance as a 2 micron thick layer of n-GaN. In a device with a conventional contact formed on n-GaN, nearest neighbor n-contacts may be spaced about 150 microns apart. Accordingly, in a device according to embodiments of the invention with a 150 nm thick ITO conductive layer 28, the distance between openings 32 may be 150 microns. In a device according to embodiments of the invention with a 30 nm thick ITO conductive layer 28, the distance between openings 32 may be 30 microns. If the openings 32 are 3 microns in diameter spaced 30 microns apart, the surface coverage of openings 32 is about 1%.

The thickness of conductive layer 28 and low-loss material 30 depend on the individual materials used, as well as on the combination of materials.

A reflective conductive layer 34 is formed over the remaining low-loss material 30 and openings 32. Reflective layer 34 electrically connects to the p-type region 26 through openings 32 and conductive layer 28. Reflective layer 34 may be, for example, silver. Reflective layer 34 may be a multi-layer stack and may include, for example, one or more reflective metals, on or more Ohmic contact metals, and one or more guard metals or other guard materials. One example of a reflective layer 34 is a stack of silver, nickel, silver, then a guard metal such as TiWN, which may prevent or reduce electromigration of the silver.

The light extraction and hence performance of the LED may be improved by adding lossless scattering to the contact. In some embodiments, p-type region 26 is grown under conditions which create a rough surface, which may improve scattering over a smooth surface. Conductive layer 28 may be formed over the rough surface as a conformal layer, which would therefore also have a rough surface. A transparent low-loss layer 30 is then formed, for example by spin-coating, to cover the roughness and create a smooth surface. Openings are formed in the low-loss material and a reflective layer 34 is then formed, as described above. The roughness of the p-type region 26/conductive layer 28 interface may cause increased loss as compared to a smooth interface, but increased extraction may result in an overall improved performance. In some embodiments, scattering is increased by making low-loss layer 30 and/or conductive layer 28 porous or columnar in structure, for example by forming an ITO conductive layer 28 and/or a SiO_(x) low-loss layer by oblique-angle deposition. The increase in porosity is accompanied by a reduction in refractive index which increases the reflectance of the contact. The porosity may be controlled by controlling the deposition angle, as described in “Quantification of porosity and deposition rate of nanoporous films grown by oblique-angle deposition,” Applied Physics Letters 93, 101914 (2008), which is incorporated herein by reference.

In one example, an Al-doped ZnO conductive layer 28 is applied to a rough p-type GaN layer grown as the top layer of the p-type region 26. A low index SiO_(x) layer 30 is spin-coated over the ZnO to provide scattering at the ZnO/SiO_(x) interface and to planarize the rough ZnO layer. Openings 32 are formed, then a silver reflective layer 34 is deposited.

In some embodiments, low-loss material 30 is a multi-layer dielectric stack. The layers closest to the conductive layer 28 and the reflective layer 34 are chosen for good adhesion. The inner layers are chosen for minimum refractive index. Multiple layers may be formed, for example, in a single processing step. A multi-layer low-loss structure 30 may be more reliable and more reflective than a single layer. In addition, the difference in index of refraction between the layers in a multi-layer stack may provide scattering, particularly when a multi-layer stack is applied to a rough surface, as described above.

In some embodiments, the etch depth of the openings 32 in the low-loss material 30 is increased to include removal of some of the underlying conductive layer 28, as illustrated in FIG. 4, which may improve the electrical contact and may increase scattering. In some embodiments, the sidewalls 33 of openings 32 are angled to provide for more optimal scattering of high angle light towards the extraction surface (i.e. the surface of the device opposite reflective material 34). The sidewall angle θ may be, for example, between 5 and 50 degrees with respect to the surface normal.

FIG. 3 illustrates an LED 42 connected to a mount 40. Before or after forming the p-contact 48, which includes conductive layer 28, low loss material 30 and reflective material 34, as described above, portions of the n-type region are exposed by etching away portions of the p-type region and the light emitting region. The semiconductor structure, including the n-type region 22, light emitting region 24, and p-type region 26 is represented by structure 44 in FIG. 3. N-contact 46 is formed on the exposed portions of the n-type region.

LED 42 is bonded to mount 40 by n- and p-interconnects 56 and 58. Interconnects 56 and 58 may be any suitable material, such as solder or other metals, and may include multiple layers of materials. In some embodiments, interconnects include at least one gold layer and the bond between LED 42 and mount 40 is formed by ultrasonic bonding.

During ultrasonic bonding, the LED die 42 is positioned on a mount 40. A bond head is positioned on the top surface of the LED die, often the top surface of a sapphire growth substrate in the case of a III-nitride device grown on sapphire. The bond head is connected to an ultrasonic transducer. The ultrasonic transducer may be, for example, a stack of lead zirconate titanate (PZT) layers. When a voltage is applied to the transducer at a frequency that causes the system to resonate harmonically (often a frequency on the order of tens or hundreds of kHz), the transducer begins to vibrate, which in turn causes the bond head and the LED die to vibrate, often at an amplitude on the order of microns. The vibration causes atoms in the metal lattice of a structure on the LED 42 to interdiffuse with a structure on mount 40, resulting in a metallurgically continuous joint. Heat and/or pressure may be added during bonding.

After bonding LED die 42 to mount 40, the growth substrate on which the semiconductor layers were grown may be removed, for example by laser lift off, etching, or any other technique suitable to a particular growth substrate. After removing the growth substrate, the semiconductor structure may be thinned, for example by photoelectrochemical etching, and/or the surface may be roughened or patterned, for example with a photonic crystal structure. A lens, wavelength converting material, or other structure known in the art may be disposed over LED 42 after substrate removal.

Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described. 

1. A device comprising: a semiconductor structure comprising a III-nitride light emitting layer disposed between an n-type region and a p-type region; a contact disposed on the p-type region, the contact comprising: a transparent conductive material in direct contact with the p-type region; a reflective metal layer; an transparent insulating material disposed between the transparent conductive layer and the reflective metal layer; and a plurality of openings in the transparent insulating material, wherein the transparent conductive material is in direct contact with the reflective metal layer in the plurality of openings.
 2. The device of claim 1 wherein the transparent conductive material is one of silver and aluminum.
 3. The device of claim 2 wherein the transparent conductive material comprises a plurality of discrete regions that occupy less than 10% of an area of a surface of the p-type region on which the contact is disposed.
 4. The device of claim 1 wherein the transparent conductive material has a thickness between 0.5 and 10 nanometers.
 5. The device of claim 1 wherein the transparent conductive material is an oxide and has a thickness between 30 and 1000 nanometers.
 6. The device of claim 1 wherein the transparent conductive material is one of indium tin oxide, nickel oxide, and ZnO.
 7. The device of claim 1 wherein the transparent insulating material is one of SiO_(x), SiN_(x), MgF₂, and Al₂O₃.
 8. The device of claim 1 wherein the transparent insulating material has a thickness between 200 and 500 nm.
 9. The device of claim 1 wherein the openings have a width between two and fifteen microns.
 10. The device of claim 1 wherein the openings are spaced between 20 and 200 microns apart.
 11. The device of claim 1 wherein the transparent insulating material comprises a multi-layer stack.
 12. The device of claim 11 wherein: a first layer in the multi-layer stack is in direct contact with the transparent conductive material; a second layer in the multi-layer stack is in direct contact with the reflective metal layer; and a third layer disposed between the first and second layers has a lower index of refraction than the first and second layers.
 13. The device of claim 1 wherein the reflective metal layer comprises silver.
 14. The device of claim 1 wherein a surface of the p-type region in direct contact with the transparent conductive material is rough.
 15. The device of claim 1 wherein the openings extend into the transparent conductive material.
 16. The device of claim 1 wherein the openings have a sidewall angle between 5 and 50 degrees with respect to a normal to a top surface of the reflective metal layer.
 17. The device of claim 1 wherein at least one of the transparent insulating material and the transparent conductive material is porous. 